This invention relates to an apparatus for converging three electron beams accurately on a picture tube screen, and more particularly relates to a dynamic convergence apparatus for a color television reproducing system which utilizes a densely doted cathode ray tube and needs an accurate convergence of the electron beams from three guns, or for a color monitor system to which a horizontal drive pulse and a vertical drive pulse are provided separately from color component signals.
To converge the beams on the picture tube screen after adjusting them at the center of the picture screen by carrying out static convergence, the parabola currents which flow through the convergence coils are caused to be such that the amplitude and amplitude balance between the first half and the second half of the scanning period are controlled, respectively. According to the conventional convergence apparatus, pulses which are supplied by a winding of a deflection output transformer or a flyback transformer, and sawtooth voltages which are produced by integrating the pulses by means of inductors are supplied to the convergence coils, and they are integrated by the self-inductances of the convergence coils to produce the desired tilt controlled parabola currents. There has also been proposed a convergence apparatus which derives the sawtooth voltages from the deflection coil and amplifies them before supplying them to the convergence coils.
In a color monitor which operates also in a state of under scanning and a color television reproducing apparatus which utilizes a densely dotted picture tube or a picture tube with a wide deflection angle, it is necessary to control the wave shape of the convergence currents more accurately. But is is difficult to converge the three beams accurately especially in the corner portions of the picture screen because of the distortion at the top end portion of the sawtooth voltage waves which are supplied to the convergence coils and the time lag of the convergence fields.